1. Field of the Invention
This invention relates to the field of noble metal alloys and to their use in the manufacture of dental restorations, and more specifically relates to a new and improved high palladium alloy containing gold, and silver, which does not discolor porcelain, has an excellent combination of physical properties for use in combination with porcelain and can be readily worked at melting temperatures without the need for special shielding atmospheres.
2. The Prior Art
As is well known, dental casting alloys should provide a high degree of biocompatibility or inertness to the conditions in the mouth and good physical properties so that they will provide long lived usage. In addition, those alloys which are used to provide castings upon which porcelain coatings may be applied must provide good bonding characteristics to the porcelain coatings, with little or no loss of strength at porcelain firing temperatures and other characteristics which are compatible with the porcelain coatings, such as similar coefficient of thermal expansion, avoidance of discoloration of the porcelain, etc. Lastly, the alloy should process well during casting and be useful with commercially available porcelains.
Previously, gold alloys, usually gold/platinum alloys, were preferred as dental casting materials for porcelain applications because they have provided a highly desirable balance of properties. The commercially available dental porcelains have been formulated so as to be compatible therewith.
Over the years, much effort has gone into developing alloys for dental applications with higher strength and lower cost (more readily available) metals. For example, cobalt base alloys almost completely displaced gold alloys in the area of partial dentures in the mid 1930s. And, more recently, nickel base alloys have made a significant impression in porcelain substrate alloys.
Within the noble metals area, much effort has been expended to use the nobility, strength and lower cost of palladium as the base for new alloy systems. In the 1930s, high palladium and palladium based alloys were attempted, but the need to deoxidize palladium was not recognized. The problems with high palladium alloys was believed, until recently, to be hydrogen dissolved therein.
A number of gold/palladium and palladium/silver alloys have been developed which provide a high degree of compatibility with porcelain and satisfactory physical and mechanical properties. However, silver has a tendency to migrate at the porcelain firing temperature and to discolor the porcelain. This silver migration and its discoloration of porcelain make porcelain selection and porcelain firing control critical in order to produce aesthetic porcelain fused to alloy restorations, particularly for anterior use.
While reduction or elimination of silver content minimized the discoloration effect, it made control of the alloy thermal expansion (contraction) more difficult. Elimination of gold has had the same effect on thermal expansion characteristics. Nevertheless, silver and gold are elements which are very useful in formulating alloys with the desirable coefficient of thermal expansion for compatibility with porcelain, about 13.8-15.times.10.sup.-6 in/in/.degree. C. In one of co-applicant's prior applications, Ser. No. 174,749 filed Aug. 4, 1980, now U.S. Pat. No. 4,350,526, there is disclosed a palladium alloy which has overcome the discoloration problem.
However, even the palladium alloys of co-applicant's patent require close control in the porcelain firing step and selection of the porcelains used in connection therewith. The generally available dental porcelains were formulated for use with high gold content alloys so as to exhibit a coefficient of thermal expansion which is typically 5-10 percent lower than the high gold content alloys This results in placing the porcelain coating in compression after cooling from the firing temperature, thereby producing a stronger restoration when it is subjected to tensile loading.
The reduction or elimination of the gold content in some of the substitute alloys has caused difficulty in maintaining a sufficiently high thermal coefficient of expansion, which is desirably in the range of 13.8-15.times.10.sup.-6 in/in/.degree. C. As indicated in co-applicant's above identified patent, silver has been used to replace gold in an effort to provide a suitable coefficient of thermal expansion but it tends to migrate at the porcelain firing temperatures, and to cause a distinct uncontrolled discoloration of the porcelain which is aesthetically unacceptable.
Another more recently developed high palladium alloy is one essentially containing both gold and silver at about 6 weight percent gold, 6.5 weight percent silver and no copper or cobalt. Because of the relatively low silver content, the alloy produces a light grey oxide which does not discolor porcelain on firing. Although the gold content is also rather low, it does enhance the alloy's properties to some extent, but the alloy is still characterized by an inadequate coefficient of thermal expansion, often causing cracking on porcelains fired thereon. As with most prior art palladium alloys, moreover, this gold and silver containing alloy is subject to considerable oxygen absorption in the molten state so that precautions must be taken to shield the molten alloy from the atmosphere during high temperature processing and fabrication.
Alloys for use as ceramo-metal restorations must also exhibit a desired balance of physical and mechanical properties. To properly support the fragile porcelain layer or the restoration, the alloy must have a yield strength, at 0.1 percent offset, of over 40,000 psi. In addition, the alloy needs high temperature strength to withstand the forces applied to the restoration while the porcelain is being fired in place.
While standard tensile tests are possible at porcelain firing temperatures, 950.degree.-1000.degree. C., a more reliable test of strength for the special circumstances of porcelain fired to metal dental restorations is the "sag" test. This test is performed on a strip of alloy 1.times.10 mm in cross section and 50 +mm long. The strip is supported on knife edge supports 50 mm apart and a static load applied. The assembly is placed in a standard dental porcelain firing furnace and heated in the same manner as a normal dental restoration. The amount of deflection is measured and this "sag" is an indication of the high temperature strength of the alloy. Sag in the 5 mm range is unacceptable. Sag of from 1-5 mm requires special precautions be made to prevent detrimental sag when a three or more unit bridge is being fired. The desired value is less than 1 mm.
Moreover, a dental casting alloy must be able to be soldered before the porcelain firing cycle. Since porcelain is fired at approximately 1000.degree. C., the alloy must possess a solidus above about 1100.degree. C. to allow the solder to flow without starting to melt the casting. However, in order to allow the alloy to be cast with standard equipment found in dental laboratories, the liquidus temperature must not be greater than about 1400.degree. C. Lastly, the alloy must also exhibit good bonding to dental porcelains.
Many palladium based and high palladium content alloys may meet the physical and mechanical requirements noted but are completely unusable due to a certain characteristic of palladium. Palladium has a high affinity for oxygen, and much of the early failure to develop high palladium alloys was the failure to recognize this problem.